The HLA-D region of the human main histocompatibility complex (MHC) contains genes for at least three different types of Ia-like molecules (DR-, MB-, and SB-like). Results of protein chemistry and molecular genetics suggest there are at least 5 genes for alpha chains and 7 genes for beta chains of human Class II histocompatibility molecules. This region also codes for products that restrict antigen-specific responses, are associated with susceptibility for development of various diseases and determine the rejection of organ allografts. The purpose of these studies is to investigate the relevant antigenic epitopes on each of HLA-D region discovered molecules, to attempt to determine if there is specialization of restriction determinants carried by human Class II molecules recognized by T cells specific for antigens of limited heterogeneity, to study the role of density of expression of Class II molecules on human antigen-presenting cells and to explore the correlation between certain HLA-D region haplotypes and the frequency of responding helper and suppressor T cells. Human donors typed with the reagents of the Ninth International Histocompatibility Workshop will be used for these studies. A variety of monomorphic and polymorphic monoclonal murine antibodies will be used to characterize the different human Class II molecules. The reactivity of each monoclonal antibody on B cells obtained from the donors to be used in our studies will be assayed by immunofluorescence and fluorescence inhibition under capping conditions. T cell lines cloned by limiting dilution will be used, in conjunction with monoclonal antibodies and deletion mutant cells, to map the library of epitopes that stimulate allogeneic T cells and those that restrict antigen-specific T cell clones. Limiting dilution will be used to determine the frequency of T cells proliferating in response to synthetic polypeptide antigens L-glutamic acid60, L-alanine30, L-tyrosine10 (GAT) and poly-L-(tyrosine, glutamic acid)-poly-D,L-alanine--, poly-L-lysine ((T,G)-A--L) and to attempt to identify high and low responder phenotypes associated with certain Class II alleles. This work will clarify the role of HLA-D region products in the regulation of human T cell functions and may lead to new approaches for the investigation of relationships of these antigens with susceptibility to various diseases.